Peri‐Siberian Ordovician to Devonian Tectonic Switching in the Olkhon Terrane (Southern Siberia): Structural and Geochronological Constraints

Abstract

The Olkhon Terrane is thought to preserve a record of the initial collision of the Siberian Craton with its peripheral orogenic system during the early Paleozoic. However, the related tectono‐metamorphic process and its time‐scale remain obscure. To address this issue, new structural observations combined with U‐Pb zircon and monazite and 40Ar/39Ar biotite geochronology were conducted in the migmatitic‐granitic Anga‐Sakhurta Zone of the terrane. An earliest syn‐collisional event associated with the development of a c. 500–480 Ma sub‐horizontal migmatitic fabric is confirmed. This early fabric was affected by later extensional doming in association with emplacement of partial melts (in terms of c. 470–445 Ma granite sills) parallel to the sub‐horizontal mechanical anisotropy. Subsequent upright folding leading to amplification of extensional domal structures and heterogeneous vertical transposition of composite horizontal fabric occurred soon after the doming, as indicated by intrusions of residual melts into the axial planes of the upright folds. The latest episode of deformation was marked by development of greenschist‐facies sinistral shear zones surrounding the Anga‐Sakhurta Zone at c. 420–400 Ma. An updated tectonic model involving (a) Middle–Late Ordovician crustal thinning associated with horizontal crustal flow, (b) Silurian crustal shortening related to northwards movement of Cambrian magmatic arc of the Birkhin Complex to the south, and (c) Early Devonian lateral extrusion and sinistral shearing associated with progression of the Birkhin Complex promontory, is proposed. Results from this study shed lights on the collisional evolution of peri‐Siberian orogenic system during the early stage evolution of the Central Asian Orogenic Belt.